The Times reported on Monday, quoting unidentified sources, that .U.S. President Joe Biden will warn British Prime Minister Boris Johnson not to renege on the Northern Ireland Brexit deal when they meet for the first time at the G7 summit this week.

Preserving the delicate peace in Northern Ireland without allowing the U.K. a back door into the EU’s markets through the 310-mile (500 km) Irish land border was one of the toughest issues of the Brexit divorce.

The British-run region remains deeply split along sectarian lines 23 years after a peace deal brokered by the United States largely ended thirty years of bloodshed. Several Catholic nationalists aspire to unification with Ireland while Protestant unionists want to stay part of the U.K.

Britain and the EU tried to solve the border riddle with the Northern Ireland Protocol of the Brexit agreement, which keeps the province in both the U.K.’s customs territory and the EU’s single market.

However, unionists say it contravenes the 1998 peace deal and London has announced that the Protocol is unsustainable in its current form after supplies of everyday goods to Northern Ireland were disrupted.

Biden is set to use a meeting with Johnson on Thursday to expressly voice his country’s support for the Protocol. He will also warn that the prospects of a U.S. trade deal with London will be damaged if the situation remains unresolved, according to The Times.

The U.S. President will also make clear to the EU that he expects it to stop being “bureaucratic” and adopt a more flexible approach to the implementation of the agreement, the paper added.

Lucy Frazer, a government minister as Solicitor General for England and Wales, told Sky News that David Frost, Johnson’s Brexit negotiator, is seeking to resolve the issues over the Brexit deal but the situation on the ground in terms of trade is “very difficult”.